If we have poor airflow here—that means we have low O2 hgh CO2, and so hopefully under thiscondition the high CO2 would eventually open the bronchial but if due to an occlusion it wouldn’t, but this diagram just demonstrates that if we have low O2 coming in and we get a constriction and that will actually cause a decrease in CO2 as well-- so it depends which decreases first. Is it the blood flow that decrease or the air flow that decreases? Remember one will lead to the other. If we have low ventilation it results in low perfusion,and as seen ventilation and perfusion increase to the rest of the lung.LECTURE FOUR: GAS EXCHANGE IN ALVEOLI AND TISSUEDiffusion-fick’s law Diffusion rate = surface area X concentration gradient X membrane permeabilityMake sure to know Fick’s law of diffusion…Surface area=very important in emphysema we have a huge loss of surface area- greatly affectsgas diffusion..Concentration gradient: gas diffuses from high partial pressure to low.Membrane permeability: affects ability of gas to diffuse across membrane. If we increase membrane thickness we decrease diffusion—this is extremely important. Oxygen exchange at the alveolar-capillary interface—have to then transport it to the tissue levels in our systemic circulation, and then we get O2 exchange at cells so O2 comes into the cell and metabolism causes the production of CO2, CO2 needs to removed so it diffuses into the blood and travels back to the lungs and we get an exchange at the alveolar-capillary interface.If there is no O2 in solution- blood, then we bring O2 into space and we get diffusion.Oxygen dissolves in this fluid. So solubility of a gas in solution determines amount of gas which will dissolve in solution at a given partial pressure. Remember this is solubility; partial pressure is at equilibrium between a gas and a liquid, individual gases flow from high partial pressure to lower partial pressures. Temperature also affects the solubility of gases and liquids as temp goes up solubility increases. Concentration is dependent upon the partial pressure and the solubility.Solubility of a gas in solution determines the amount of gas which will dissolve in the solution (at a given partial pressure).Partial pressure is at equilibrium between a gas and a liquid. Individual gases flow from high partial pressures to lower partial pressures.TemperatureConcentration is dependent upon the partial pressure and the solubility.Note that concentration of O2 in liquid phase (at equilibrium) or blood is significantly lower thanin the air..only .15mmol/L O2 has low solubility in liquid in this case in blood…this creates problem—how do we get O2 diffusing into our blood so our tissues can use it? This is where RBCs come in to help transport them.Know how partial pressure of O2 behaves between the gas and the liquid phase.CO2 has a higher solubility in a liquid (less CO2 leave) If we increase partial pressure we increase concentration.O2 has poor solubility which is why we have hemoglobinWhen we talk about O2 diffusion and we are talking about the Partial pressure--goes to equilibrium—because we get a partial pressure of 100mmHg in the alveoli we therefore get a partial pressure of O2 in the pulmonary capillaries-100mmHg.. As blood flows by these alveoli it goes into the left side of the heart and then goes out the body--**O2 being delivered to the body has the same partial pressure of the alveoli, as we get to the tissues they consume O2 due to metabolism so their partial pressure is lowered, O2 diffuses from high partial pressure to low so the it diffuses into the tissues. And when it does this we lose partial pressure—it drops and since concentration is proportional to partial pressure, it drops as well. So under normal circumstances blood returning to the heart is at a partial pressure of 40mmHg. **remember what these normal Partial pressures are in the arterial circulation—100mmHg and in the venous systemic system—40mmHg CO2 is produced in the tissues so we are going to start at the tissue level. The tissue produce CO2 as a result of metabolism—PCO2 is about 46mmHg so that’s related to concentration—so concentration goes up. Because alveoli have a pressure of 0 itdiffuses and dilutes with that. This is slow b/c remember every time we take a breath, we don’t due a full exchange because of the dead space. Equilibrium pressure of 40mmHg in the alveoli which then goes to a new equilibrium in the arterial circulation and goes back to the tissues and picks up more CO2 and comes up to 46mmHg REVIEW THESENormal pH in arterial—7.4 and in venous—7.37 so venous is slightly more acidic because there is more CO2, which when it is transports dissociates to a hydrogen ion which adds to the acidity and decreases pHREMEMBER normal values100mmHgHypoxia typically goes hand in hand with hypercapnia (elevated carbon dioxide levels).Deviations from normal and causes:Hypoxic: low arterial PO2 usually caused from high altitude because since the pressure in the altitude is lower, this effects everything else since now the pressure in the alveoli has to be lower in order to have the diffusion from high to low. Alveolar hyperventilation; as we decrease our alveolar ventilation we bring in less fresh air, we still keep consuming O2 so it decreases PP of arterial O2. Decreased lung diffusion capacity such as fibrotic lung disease, emphysema, these diseases decrease our lung diffusio capacity—think of ficks law. Abnormal ventilation-perfusion ratio: remember we said we like to match ventilation to perfusion, if for some reason these aren't well matched this can decrease PP of O2Anemic hypoxia caused by: Carbon monoxide poisoning—CO2 binds to hemoglobin much morereadily than O2 does therefore decreasing the amount of O2 bound to hemoglobin. Histotoxic hypoxia: tissues are now hypoxic because they cannot utilize O2….cyanide poisons the metabolic capacity of the tissues to use O2Hypoxia typically goes hand in hand with hypercapnia—elevated carbon dioxide levelsLow O2 typically means high CO2Po2 is determined by atmospheric pressure and alveolar ventilation. This is in reference to primarily fick’s law. So remember in a normal lung we said had normal partial pressure of O2 in alveoli which should be 100mmHg it goes to equilibration to blood circulation which is also 100mmHg. But in this case the PPO2 is determined by atmospheric pressure Also dependent on alveolar ventilation: if we